1. Field of the Invention
The present invention relates to a vibration damping apparatus for damping vibrations produced in pulse motors such as linear pulse motors sand rotary pulse motors for intermittently driving and feeding a head of a floppy disk regeneration device and the like.
2. Description of the Prior Art
Pulse motors can be positioned by closed loop control and hence are widely employed for various types of driving mechanisms in office automation terminal apparatuses. For example, with the recent progress of office automation, floppy disk regenerative devices have prevailed as memories. The head of such a floppy disk regenerative device is frequently driven by a pulse motor. Moreover, pulse motors are employed for driving print heads of printers and typewriters, etc. These pulse motors, however, suffer from some problems. In particular, vibration produced in such a pulse motor causes severe problems for various applications. In the following, a general arrangement of a pulse motor will first be described, before discussing the problems.
FIGS. 12 and 13 exemplarily show a prior pulse motor of this type.
In these figures, designated at 1 is a mild steel movable member (primary member), and 2 is a stator (secondary member). The movable member 1 has a comb-shaped movable tooth 1a formed on the lower surface thereof. The stator 2 includes a permanent magnet 3, a slit plate 4, electromagnet coils 5, a yoke 6, linear bearings 7, guides 8 and 9, and a back plate 18. The slit plate 4 is disposed on the lower side of the movable member 1 in a confronting relation therewith. The yoke 6 around which the electromagnet coils 5 are fixed, and the flat H-shaped permanent magnet 3 are fixedly mounted in or on the lower side of the slit plate 4. The yoke 6 has in the center thereof, a cross-shaped groove to thereby form four magnetic poles 6a, 6b, 6c, and 6d. The slit plate 4 has comb-shaped pole teeth 4a, 4b, 4c, and 4d formed at positions just above the magnetic poles 6a, 6b, 6c, and 6d. These pole teeth 4a, 4b, 4c, and 4d are positioned so as to be separated from the movable tooth 1a of the movable member 1 by a very minute gap in a confronting relation therewith by holding the linear bearings 7 between the slit plate 4 and the movable member 1. The guides 8 and 9 are mounted on the upper side of the movable member 1. The linear bearings 7 and the movable member 1 are fitted longitudinally movably in a travelling path formed by the slit plate 4 and the guides 8 and 9. The back plate 18 is mounted on the lower side of the permanent magnet 3.
With the arrangement as described above, the movable member 1 is mounted movably longitudinally thereof relatively to the stator 2. In addition, the movable tooth 1a and the pole teeth 4a, 4b, 4c, and 4d are adapted oppose to each other across a minute gap. With a pulse-shaped current supplied to the coils 5, in this situation, the movable member 1 is moved stepwise on the stator 2.
Since the prior linear pulse motor described above is driven stepwise by the pulse-shaped current as described above, the movable member 1 is forced to vibrate longitudinally thereof for some time around a prescribed position even after it is moved to that position. In particular, a pulse motor does not need a conversion mechanism for converting a rotating motion to a linear motion for example between it and an object to be driven since it undergoes the linear motion by itself, but this unfortunately causes the time needed to settle the movable member 1 to be prolonged since the vibration of the movable member 1 is not cancelled out by making use of mechanical loss of the conversion mechanism. Therefore, when the pulse motor is employed for driving the head of a floppy disk, the time to access data is delayed or the data so accessed presents insufficient reliability. Moreover, when the pulse motor is employed for driving print heads of printers and typewriters, etc., the printing speed thereof can not be increased, or an increase, if possible, of the printing speed results in deterioration of printing quality. In addition, another problem with such a pulse motor will be described. A linear pulse motor is in general rotated followed by vibration since it is rotated stepwise through a prescribed angle (step angle) for each pulse input. In addition, such a motor can be regarded as a kind of spring from viewpoint of the mechanism of producing the torque of the motor, and so the vibration of the motor is accelerated and amplified by stopped pulsating torque to result in further larger vibration. Accordingly, such vibration causes severe problem depending on the applications of the pulse motor and hence it is urgently desired to prevent such vibration from being produced. There are known conventional methods to prevent such vibration such as an electronic method wherein the timing of a motor driving pulse is controlled and a mechanical method wherein a frictional load is mounted on a motor shaft or both the frictional load and an inertia load are mounted thereon.
FIG. 14 is a cross sectional view illustrating a prior arrangement of a vibration damping apparatus 57 using the frictional load and the inertia load in the mechanical method described above. In the figure, designated at 51 is a pulse motor, and 52 is a shaft thereof. A mild steel disk 53 having a mounting part 53 a is attached to the shaft 52 by means of a screw 54, which disk includes an annular frictional plate 55 bonded thereto. An annular inertia member 56 composed of a permanent magnet 56a and a back plate 56b is disposed on the other surface of the frictional plate 55 in contact therewith. This back plate 56b forms magnetic paths .theta., between the permanent magnet 56a and the mild steel disk 53 to serve as a flywheel for averaging variations in the rotating speed of the motor. These mild steel disk 53, frictional plate 55, and inertia member 56 together constitute the vibration damping apparatus 57.
With the pulse motor 51 being driven by such arrangement, the inertia member 56 and the mild steel disk 53 are integrally rotated owing to the magnetic force of the permanent magnet 56a when the acceleration of the motor 51 is low. On the other hand, when higher acceleration is applied to the pulse motor 51 (upon acceleration or interruption), the inertia member 56 is left behind the rotation of the shaft 52 because of the inertia force thereof to produce sliding thereof. Owing to this sliding, excess kinetic energy is converted to thermal energy, whereby the rotation speed of the pulse motor 1 is prevented from becoming uneven and the pulse motor 1 is prevented from vibrating during an interruption in the rotation thereof.
However, it is inherent in the prior vibration damping apparatus described that heat is produced by permitting the inertia member 56 to largely slide for assuring a satisfactory effect of the vibration prevention. In order to dissipate this heat it is necessary to increase the size of the back plate 56b while providing the permanent magnet 56a having the attraction force in a size corresponding to the size of the plate 56b. As a result, the vibration damping apparatus 57 becomes large-sized and the weight thereof is increased, which results in deterioration of the acceleration characteristic of the pulse motor 1.